The invention relates generally to offshore seismic prospecting and, more particularly, to apparatus and methods for determining the position and shape of hydrophone streamer or bottom cables towed or dragged underwater behind seismic survey vessels.
In offshore seismic prospecting, a group of underwater cables, commonly referred to as streamers, is towed through the water by a marine vessel, such as a surface ship. Each streamer is instrumented with an array of hydrophones used to receive seismic reflections off undersea geologic formations and with a variety of sensors and control devices used to determine the shape and position of the streamers and to control their depth. The surface ship also tows a seismic source, typically an array of air guns fired at regular intervals to radiate seismic energy into the geologic formations. For an accurate survey, the shape and position of the streamers and the seismic source must be known accurately. Conventionally, depth sensors, acoustic ranging transceivers, and magnetic compasses are deployed along the streamers to provide data from which the relative shapes of the streamers can be computed on board the vessel or when the seismic data are processed. Head and tail buoys are tethered to the head and tail ends of the streamers as visual markers. Acoustic ranging transceivers at the surface ship and on the seismic source combine with the transceivers at the head end of the streamers to form an acoustic network to tie the positions of the head ends of the streamers to the ship and the seismic source. Acoustic ranging transceivers on the tail buoys and at the tail end of the streamers form a tail-end acoustic network. GPS (Global Positioning System) receivers on the ship, on the seismic source, and on the tail buoys at the tail ends of the streamers help tie the positions of the head and tail ends of the streamers, the seismic source, and the ship to absolute geodetic positions. A complex navigation system is used to estimate the sensor positions and thereby the shapes of the streamers from the many depth, range, and heading observations made in the entire streamer spread.
While these conventional cable positioning systems work well, they do have some shortcomings. For example, the acoustic ranging transceivers take a lot of power to transmit acoustic pulses of sufficient energy to achieve quality long ranges. This power requirement shortens battery life for battery-powered devices and puts constraints on those devices powered inductively from the streamers. Furthermore, in-line ranges (i.e., ranges along the length of the streamer) are difficult to achieve especially if the acoustic transducers are mounted within, rather than suspended from, the streamers. And in-streamer compasses are negatively affected by their proximity to metal strength members and conductors running through the streamers. But when these devices are suspended from, rather than built into, a streamer, they must be removed before the streamer is wound up on the storage reel and attached again as the streamer is payed out. This removal and attachment interrupts streamer retrieval and deployment. The navigation system, moreover, only estimates the positions of the sensors from a model of the streamer and from the many observations. Because many of the position estimates are reckoned from one or more other estimated positions, position errors can build up along the streamers. This problem of position error propagation is especially troublesome in longer streamers.
As these shortcomings suggest, there is a need for a streamer positioning system that can provide benefits such as low power consumption, minimal interruption to streamer retrieval and deployment, and accurate streamer positioning.
The use of GPS for navigation is widespread and continues to develop. Because the GPS satellites transmit radio frequencies, which cannot penetrate water, they are not directly useful in underwater applications. But there have been efforts to extend GPS to underwater use. For example, U.S. Pat. No. 5,119,341, “METHOD FOR EXTENDING GPS TO UNDERWATER APPLICATIONS,” issued Jun. 2, 1992, to Youngberg describes a system that provides buoy-mounted beacons that dynamically determine their positions from GPS satellite signals and broadcast data representing their positions acoustically to underwater vehicles. In another system, the NASNet™ system sold by Nautronix Ltd. of Fremantle, Australia, a network of stations is placed on the sea floor at surveyed locations. Once in place, the stations acoustically transmit time-related GPS-like information to one or more receive-only users, which can use the information in much the same way as a GPS receiver does with GPS data.
But, in both these “underwater GPS” systems, the acoustic transmitters are immobile or drifting and cover a fixed or slowly changing volume of the ocean. To be used in another part of the ocean, such as for a new seismic survey, another such network would have to be set in place. For this reason, both of the systems described have shortcomings for use in streamer positioning in offshore seismic prospecting.